This invention relates to the art of sonar target simulation and more particularly to the realistic simulation say of a submarine as a target to train sonar operators, test sonar equipment, or for operational decoy purposes.
Submarine targets have been simulated heretofore by motorized target devices, generally about the size and configuration of a conventional submarine torpedo, which carry electronic means for receiving a sonar ping signal, modifying the signal to have characteristics of a reflected ping, and then transmitting the modified signal so that when received and processed by the tracking sonar it will resemble an actual submarine target echo. One example of such a device is given in U.S. Pat. No. 3,180,295.
Various systems and methods have been devised for such modifying of an active sonar signal to "look" like a truly submarine reflected echo signal. Among the echo features simulated are target length, target reflective highlights, and target aspect angle. Electronic systems for accomplishing these are exemplified by U.S. Pat. 3,641,485 and copending applications for patent Ser. No. 94,134, filed Dec. 1, 1970, now U.S. Pat. No. 3,671,657, and Ser. No. 134,558, filed Apr. 16, 1971, now U.S. Pat. No. 3,713,081. While the techniques described therein provide very effective results in simulating submarine echoes to the sonar equipment that is doing the searching or tracking, for the reasons about to be discussed the cooperation of a second sonar equipped ship which receives the simulated echoes can result in a determination that the echoes are from something other than an actual submarine, e.g., a decoy.
The duration of an echo returned by a lone, narrow target such as a submarine, measured on a line connecting the target and the illuminating sonar, is approximately 2s/c.vertline.cos .theta.hd i.vertline.+T seconds, where s is the target length, c the velocity of sound, .theta..sub.i the angle between the target axis and the direction of the sound source, and T is the pulse length. Measured along any other line (such as in a direction toward a listening ship), the echo length is s/c (cos .theta..sub.i +cos .theta..sub.r)+T, where .theta..sub.r is tile angle made by the reflected sound in the direction of tile receiver with the target axis (see Urick, "Principles of Underwater Sound for Engineers", p 26).
Echoes having a length of 2s/c.vertline.cos .theta..sub.i .vertline.+T seconds may be generated using several techniques some of which are described in the aforementioned patent and applications. When radiated front a buoy or torpedo in response to an illuminating pulse, these small targets appear to the sonar transmitting the pulse as an actual submarine target. To a sonar observer on a second ship, however, it will be noticed that the echo length is a function only of .theta..sub.i, i.e., it does not change with changes in position bearing of this second ship relative to the target. The reason for this is that the "echo" is radiated from a single projector--essentially a point source at the mobile simulator. By means of a radio link between the two ships, the echoes can be compared and the fact that their lengths remain approximately the same used to distinguish this simulated echo from real submarine echoes that may be present.